Attrebus was a fun character to read about in the first book. I think that's where most of his character development lies. In the second book he mostly just seems to follow Sul around and do what he's told, and by the end of that book I found that I still did not respect him very much at all.
Sul was a mildly interesting character, but seemed far too shallow overall. Consequently, when he died I was not emotionally affected at all.
I never really liked Annaig all that much. While its true that she was forced to do many of the things she did simply to survive, at the same time you are never given much of a chance to like her in the first place because you never see her do anything in either of the books that particularly stands out as being selfless, save for the very end of the book when she allows Mere-Glim to save the trees in the Fringe Gyre and leave with them. She doesn't seem to end up very happy in the end of the book, either - the only person of interest she has left in the world is this Attrebus jerk whom even she doesn't seem to care about very much.
Colin was very, very flat. His girlfriend's betrayal was a poorly executed plot twist for multiple reasons. Needless to say, I didn't care to see either of them die - they seemed to simply be tools used by the author to show the reader how Umbriel came to Nirn.
Mere-Glim was my favorite character in the book. He is rather strange in many ways (mostly because he's an Argonian
) but his naturally selfless nature makes him really likable IMO. Retrospectively, it was really dumb of him to get himself wrapped up with the inhabitants of Umbriel - if he had kept his place and stuck with his friend Annaig to the end, then everything would've worked out fine. Nevertheless, I was really saddened at the end of the book when he chose to leave Annaig to take the trees in the Fringe Gyre home. Perhaps it was all he could do with himself at that point, considering he could no longer communicate with the Hist on Tamriel. But he doesn't seem to be particularly happy with his decision in the epilogue, and frankly, neither am I. I like to take hope in the possibility that he's still on Nirn in a place where the Hist grow unhindered (which makes the most sense IMHO - how else could these trees be related to the hist unless they come from the same realm?).